Composite materials have well-documented advantages over traditional construction materials, particularly in providing excellent mechanical properties at very low material densities. As a result, the use of such materials is becoming increasingly widespread and their application ranges from “industrial” and “sports and leisure” to high performance aerospace components.
Prepregs, comprising a fibre arrangement impregnated with resin such as epoxy resin, are widely used in the generation of such composite materials. Typically a number of plies of such prepregs are “laid-up” as desired and the resulting assembly, or laminate, is placed in a mould and cured, typically by exposure to elevated temperatures, to produce a cured composite laminate.
However, without any surface treatments applied, such composite materials often have a rough, or pinholed, surface. This tendency to form an uneven surface appears to be closely linked to the coarseness of the underlying fibres, the problem being more pronounced the coarser the fibres are. This can be a particular problem when coarse fibres are routinely used and yet a smooth surface finish is highly desirable, one example being in the construction of windblades.
A known technique for providing a smooth surface finish to such composites is to use a so-called gel coat. Briefly, this involves pasting a flowable composition on the internal surface of the mould which is then cured to form a gel layer, the prepreg assembly being placed onto the gel layer prior to curing. During cure, the prepreg and gel coat fuse together, this generally resulting in a smooth glossy surface to the formed composite. However, the process is very labour intensive requiring an initial curing step and tends to produce a gel layer of uneven thickness. Furthermore, the layer needs to be relatively thick (e.g. on average 0.3 mm) to cover up the defects, having an undesirable weight increase.
In view of the disadvantages of applying a gel coat, an alternative solution proposed involves incorporating a so-called surface finishing film into the prepreg assembly. WO 2008/007094 discloses such a finishing film, involving a surface prepregged microfibre sheet with an underlying fleece of glass fibre, also prepregged. On curing, the finishing films prevent the formation of the pinholes and provide a smooth surface without requiring a gel coat.
It would therefore appear that the problem of providing a good surface finish, even to coarse fibre composites, had largely been solved. However, hitherto unforeseen problems have now been found to arise in practice.